It is well known to provide a conveyor to move material throughout a plant. In many instances, electric motors are utilized to drive a chain which in turn is routed throughout a manufacturing line or plant so as move a part (load) along a predetermined path. The weight of the load(s) to be carried by a conveyor system are normally predetermined and are factored into a formula for ascertaining the torque requirements for a given drive motor or set of drive motors that are required to move all anticipated loads at a given speed. Sensing devices such as limit switches or proximity switches are often used to monitor and effectuate the control of the movement of loads along the established conveyor path. The loads are typically supported and carried by an element of the conveyor mechanical system commonly called a “carrier”.
A common type of conveyor system is a monoveyor which can transport a load or series of loads on carriers that are directly attached to the chain of the conveyor system. Another common type of chain conveyor is a “power & free” system. As with the monoveyor systems, the primary moving element is a chain, however, in this case the chain is utilized to push detached carriers along the path of the conveyor. These detached carriers are located in a separate “free rail”. Mechanisms along the pathway of the conveyor allow the carrier to disengage, stop, and switch to different pathways of the conveyor system. The load in either of these systems can be a work piece that needs to be delivered to various work stations in order for particular operations to be performed such as machining, and once machined, the work piece continues on the conveyor to the next station. Multi-drive systems are commonly used to deliver power and movement to these conveyors.
The problem however is that traditional multi-drive systems do not compensate well under all conditions for the changes in torque demand as loads advance along the conveyor line. This may cause the drive motor in any given zone along the conveyor line to operate inefficiently. For example, a drive motor at a particular zone that is temporarily carrying a substantial portion of the total load could be under performing by not producing sufficient torque output so as to move the work piece given the dynamic conditions of the conveyor line. By contrast, a different drive motor in another zone that is temporarily carrying very little load could be over performing and producing too much torque output thus causing the conveyor chain and thus the carriers and loads to advance along the conveyor line erratically. This inconsistency amongst the zones can also cause undue stress on the components of the conveyor.
One method of determining the amount of torque required to be produced by a given motor or group of motors so as to properly advance a work piece on a conveyor under all expected conditions is to use a mathematical model commonly referred to as a chain pull calculation. Typically, a chain pull calculation is a static mathematical model that is utilized during the design phase of a chain conveyor. Once the pathway of the system is determined and the speed and load requirements are established, a series of static chain pull calculations are completed. Each static calculation is carried out with conveyor loads shown at various potential positions on the system. These calculations are also used to determine the optimal power delivery as well as location of the drive or drives along the path of the conveyor. When the model is utilized, drives are typically located and sized such that motors of equal horsepower are placed along the chain path. In essence, final placement and sizing of drives is based upon this mathematical model of the conveyor mechanical system and the limits imposed by the conveyor pathway that is determined in order to perform the work anticipated by that design.
Presently, a number of control technologies are employed to allow multi-drive systems to perform the required work. All systems rely to some degree on motor “slip” to balance the relative torque demands during operation. Some systems utilize torque feedback techniques to adjust individual motor torque outputs. In any case, the individual drive control systems “react” to the imposed load conditions. For this reason, chain conveyors are designed with the limitations imposed by reactive drive systems in mind. With the current available technologies employed, the longer the chain and the greater the quantity of drives utilized in a multi-drive system, the higher the likelihood of erratic performance. Therefore multi-drive conveyors are avoided when possible. The present invention overcomes this problem.
A recognized “rule of thumb” in the conveyor industry is to limit the total length of chain in any single continuous chain system to 2000 feet. This is primarily due to the stress imposed on the chain and mechanical guide components by the build up of “chain tension” that partially occurs as a result of the standard practice of maintaining a taut chain. This chain tension tends to increase, in general, as the length of the chain is increased. The chain is kept relatively taut to avoid mechanical jamming or binding and or erratic fluctuations in chain speed sometimes referred to as “surge” due to the effect of uncontrolled chain “telescoping” when proper chain tension is not maintained. Operational irregularities can result due to chain speed inconsistencies that can affect both manual and automatic systems interfacing with the conveyor and in some cases can present a safety hazard or cause negative quality impacts on product within a plant or conveyor line.
Therefore, it would be desirable to provide a conveyor drive control system that overcomes the aforementioned problems. The preferred system should be dynamic and operable to constantly change performance output of every motor within the system, in view of the constantly changing loads on the system. It would also be desirable to provide a drive control system that improves the available technology such that torque demands on the drive system are anticipated and proactively met, speed requirements are maintained, and in addition, total continuous chain length may be effectively increased well beyond the 2000 foot limitation so as to accommodate the total length of the work zone of the conveyor.
It would also be desirable to provide an improved drive control system for a multi-drive system that continuously senses the position and weight of a load or loads as they travel about the system, sends that information to a computer which in turn calculates torque requirements for each drive within the system, and then sends a corresponding signal back to a drive controller for producing the proper torque output for each drive within the materials handling a system.
It would also be desirable to provide an improved drive control system that improves delivery of balanced power and speed control throughout the multi-drive material handling system. It would also be beneficial to increase the work efficiency of these systems by reducint the amount and weight of the “return chain” of power & free systems, extending the total work length of the chain, as well as a reduction in the total number of drives required to move all loads along the path of the conveyor system so as to provide cost savings.
It will be appreciated that the improved drive control system can be employed with many types of conveyors where multiple motors are used to move one or more loads throughout a manufacturing line.
One aspect of the present invention provides a drive control system for a conveyor comprising one or more variable speed motors for driving a conveyor device. A dynamic chain pull calculation program is operable to continuously calculate the torque requirements for each motor in the system. One or more sensors are employed that are operable to create a signal indicative of a load transiting a designated zone and, if required, additional data including the weight of such a load or loads, and send data to a PLC. The PLC then converts the data from each sensor and transmits a signal to a computer operating the dynamic chain pull calculation program. A computer is operable to process the signal from the PLC using the dynamic chain pull calculation program and in turn generates real time drive torque data for each motor. A motor controller is operable to control each variable speed motor within the system so that the proper torque is generated by each motor as is required for optimum performance. The resulting drive control system is dynamic and continuously monitors torque requirements for each motor within the system given current load data so as to maximize efficiency of the conveyor.
Further areas of applicability of the present invention will become apparent from the detailed description provided herein. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It will be appreciated that the present invention can be utilized in a variety of converyor systems, and where it is desirable to efficiently transport materials through a plant or facility.